The present application claims priority to Japanese Application No. P11-268803 filed Sep. 22, 1999, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a method of producing a flat panel display (FPD) such as a liquid crystal display panel or O-ELD (organic electroluminescence display), and particularly to a method capable of producing, at a high throughput, a large-area liquid crystal display panel having a horizontal scanning circuit portion including a TFT characteristic having a high drive current (high mobility), and a pixel portion and a vertical scanning circuit portion each of which contains crystal grains excellent in uniformity.
In recent years, FPDs, in particular, liquid crystal display panels have been extensively used as display units for various kinds of electronic equipment. Currently, an active matrix type in which switching of pixels is performed by turning-on/turning-off switching elements formed at respective pixels of a display portion currently ranks as the dominant liquid crystal display panel.
With respect to such an active matrix type FPD such as a liquid crystal display panel, TFTs formed by an amorphous silicon thin film has come to be used as pixel switching elements because the amorphous silicon thin film having a good quality can be uniformly formed on the base of the panel over a large area.
The use of TFTs formed by an amorphous silicon thin film, however, has a problem. Since scanning portions such as a horizontal scanning circuit portion and a vertical scanning circuit portion of a liquid crystal display panel require high speed operation, TFTs used therefor require a TFT characteristic having a high drive current (high mobility). Accordingly, it is undesirable to use TFTs formed by an amorphous silicon thin film, which are low in operational speed, as TFTs of scanning portions of a liquid crystal display panel. To solve such a problem, liquid crystal display panels of a type of making use of an amorphous silicon thin film have been generally configured such that TFTs formed by the amorphous silicon thin film are used as pixel switching elements of a pixel portion, and a horizontal scanning portion and a vertical scanning portion, which are formed by exclusive use ICs, are externally connected to the pixel portion.
On the other hand, TFTs formed by a polycrystalline silicon thin film are higher in operational speed than TFTs formed by an amorphous silicon thin film. In this regard, there have been proposed FPDs such as liquid crystal display panels of a type in which scanning portions such as a horizontal scanning circuit portion and a vertical scanning circuit portion are formed by using TFTs formed by a polycrystalline silicon thin film and also a pixel portion is formed by using TFTs formed by the polycrystalline silicon thin film. With respect to the manufacture of liquid crystal display panels using TFTs formed by a polycrystalline silicon thin film as switching elements, there has been developed a technique of annealing an amorphous silicon thin film by irradiation of a pulse laser having an ultraviolet wavelength region such as an excimer laser, thereby crystallizing the amorphous silicon thin film.
Such a known laser annealing method generally involves irradiating an amorphous silicon thin film with a linear laser beam having a width less than 800 xcexcm by several shots at each location by moving the laser beam relative to the amorphous silicon thin film, to anneal overall the amorphous silicon thin film, thereby crystallizing the amorphous silicon thin film.
This laser annealing method, however, has a problem that since the linear laser beam having a width less than 800 xcexcm is used, not only the throughput is low but also crystal grains become uneven at adjacent laser irradiation portions.
To solve the problem caused by using a linear laser beam, there have been proposed methods of annealing overall an amorphous silicon thin film by using a large-area laser beam, thereby crystallizing the amorphous silicon thin film (for example, xe2x80x9cComparison of Effects Between Large-Area-Beam ELA and SPC on TFT Characteristicsxe2x80x9d, 1996 IEEE p1454xcx9c, and xe2x80x9c1 Hz/15 Joules-excimer laser development for flat display applicationsxe2x80x9d, SID ""99 Conference). However, it has been actually impossible to crystallize, using a large-area laser beam, a large-area amorphous silicon thin film having a size of 10 inches or more by annealing from the viewpoints of energy density required for crystallization and load of an optical system.
An object of the present invention is to provide a method of capable of producing, at a high throughput, a large-area liquid crystal display panel having a horizontal scanning circuit portion including a TFT characteristic having a high drive current (high mobility), and a pixel portion and a vertical scanning circuit portion each of which contains crystal grains excellent in uniformity.
To achieve the above object, according to the present invention, there is provided a method of producing a FPD such as a liquid crystal display panel or O-ELD, including the steps of: irradiating a location, in which a horizontal scanning circuit portion is to be formed, of an amorphous silicon thin film panel to be crystallized with a specific number of shots of a laser beam having a uniform energy density distribution and having a rectangular shape of a long-side larger than a width of the amorphous silicon thin film panel and a short-side larger than a short-side of the horizontal scanning circuit portion, in a state in which a relative positional relationship between the amorphous silicon thin film panel and the laser beam is fixed; and irradiating a location, in which a vertical scanning circuit portion and a pixel portion are to be formed, of the amorphous silicon thin film panel with the laser beam while moving the laser beam relative to the amorphous silicon thin film panel along the length direction of the amorphous silicon thin film panel.
With this configuration, since laser annealing is performed by using the laser beam having a uniform energy density distribution and having a rectangular shape of a long-side larger than a width of the amorphous silicon thin film panel to be crystallized and a short-side larger than a short-side of the horizontal scanning circuit portion, it is possible to anneal the horizontal scanning circuit portion by the laser beam without the need of moving the laser beam relative to the horizontal scanning circuit portion, and hence to prevent crystal grains in the horizontal scanning circuit portion from becoming uneven. Also, since the location, in which the horizontal scanning circuit portion is to be formed, of the amorphous silicon thin film panel is annealed by irradiating the location with the specific number of shots of the laser beam in the state in which the relative positional relationship between the amorphous silicon thin film portion and the laser beam is fixed, it is possible to produce crystal grains having large grain sizes in the horizontal scanning circuit portion, and hence to form TFTs having a high mobility in the horizontal scanning circuit portion. Further, since the pixel portion and the vertical scanning circuit portion, which do not require a TFT characteristic having a high mobility comparable to that required for the horizontal scanning circuit portion, are formed by irradiating the amorphous silicon thin film panel with the laser beam while moving the laser beam relative to the amorphous silicon thin film along the length direction of the amorphous silicon thin film panel, it is possible to produce crystal grains having grain sizes which are small and less varied while preventing the crystal grains from becoming uneven in the transverse direction of the amorphous silicon thin film panel, and hence to form TFTs having uniform threshold values in the pixel portion and vertical scanning circuit portion.
In one preferred mode of the present invention, the amorphous silicon thin film panel is irradiated with the laser beam while the laser beam is moved relative to the amorphous silicon thin film panel along the length direction of the amorphous silicon thin film panel for each shot of the laser beam, to form the pixel portion and the vertical scanning circuit portion.
In another preferred mode, the laser beam and the amorphous silicon thin film panel are moved relative to each other so that adjacent two of areas, irradiated with the laser beam continuously emitted, of the amorphous silicon thin film panel are overlapped to each other in the length direction of the amorphous silicon thin film panel.
In a further preferred mode, the laser beam and the amorphous silicon thin film panel are moved relative to each other so that adjacent two of areas, irradiated with the laser beam continuously emitted, of the amorphous silicon thin film panel are 50% or more overlapped to each other in the length direction of the amorphous silicon thin film panel.
In a still further preferred mode, the amorphous silicon thin film panel is irradiated with a laser beam having an energy density smaller than that of the laser beam used for forming the horizontal scanning circuit portion while the laser beam is moved relative to the amorphous silicon thin film panel along the length direction of the amorphous silicon thin film panel, to form the pixel portion and the vertical scanning circuit portion.
Additionally, the energy density of the laser beam may be lowered by reducing the power of the laser beam emitted from a laser light source, or optically enlarging a width in the short-side direction of the rectangular laser beam.
The width in the short-side direction of the rectangular laser beam is preferably enlarged by using a slit.
The laser beam and the amorphous silicon thin film panel may be moved relative to each other by moving the amorphous silicon thin film panel, or by moving an optical system used for irradiating the amorphous silicon thin film panel with the laser beam.
The energy density of the laser beam may be in a range of 150 to 900 mJ/cm2, preferably, 300 to 750 mJ/cm2, more preferably, 450 to 600 mJ/cm2.
In the case of using the laser beam having an energy density of 450 to 600 mJ/cm2, the specific number of shots may be in a range of 2 to 60, preferably, 5 to 40, more preferably, 10 to 30.
The laser beam may be an excimer laser beam.
The excimer laser may be selected from a group consisting of an XeCl excimer laser having a resonance wavelength of 308 nm, a KrF excimer laser having a resonance wavelength of 248 nm, and an ArF excimer laser having a resonance wavelength of 193 nm.